Detection and Quantitation The Mass Spectrum

The signal from the detector is fed to a recorder, which produces the mass spectrum. In modem instruments, the output of the detector is fed through an interface to a computer. The computer can 

FIGURE 8.16 Schematic representation of a linear channel electron multiplier (a) and a curved channel electron multiplier (b). (From Gross, J. H., Mass Spectrometry A Textbook, Spinger, Berlin, 2004. Reprinted with permission.) 

Molecules in nature do not occur as isotopically pure species. Virtually all atoms have heavier isotopes that occur in characteristic natural abundances. Hydrogen occurs largely as H, but about 0.02% of hydrogen atoms are the isotope H. Carbon normally occurs as but about 1.1% of carbon atoms are the heavier isotope C. With the possible exception of fluorine and a few other elements, most elements have a certain percentage of naturally occurring heavier isotopes. 

Through the use of electron multiplier circuits, this current can be measured so accurately that the current caused by just one ion striking the detector can be measured. These detectors are based on the simple concept of the Earaday cup, a metal cup that is in the path of ions emanating from the mass analyzer. When an ion strikes the surface of the electron multiplier two electrons are ejected. The approximately 2-kV potential difference between the opening and end of the detector 

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The signal from the detector is fed to a recorder, which produces the mass spectrum. In modem instruments, the output of the detector is fed through an interface to a computer. The computer can store the data, provide the output in both tabular and graphic forms, and compare the data to standard spectra, which are contained in spectra libraries that are also stored in the computer. 

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